Hi Yasser
Although it's not obvious, I'm pretty sure that the number of hours or days in the deadline field can actually be an xpath expression. So you could calculate the number of days/hours based on the value in the form, and use that in an xpath expression to specify the deadline.
If you need more sophisticated time calculations (eg number of business days, excluding weekends and holidays), we have a series of QPACs you can use. We also have a beta User qpac that allows you more flexibility in the way that deadlines are calculated - if you're interested, let me know.
Howard
http://www.avoka.com